The present invention relates to a method and an apparatus for removing mercury species, in particular, elemental mercury, from hot flue gas produced in a fossil-fuel energy conversion plant.
Exposure to high levels of mercury is associated with serious neurological and developmental effects in human beings. Concentrations of mercury in air are usually low and of little concern, but once mercury enters water, it can accumulate in fish and cause harm to people who eat mercury-contaminated fish. Fossil fuels contain many heavy metals, including mercury. Even if the levels of mercury in coals are low, usually between about 0.05 and 0.2 ppmw, mercury emissions from coal-fired power plants have recently been determined to pose a significant hazard to public health. Thus, the reduction of mercury in the exhaust gases of utility power plants is of great importance.
It is known that exhaust gases of fossil-fuel fired power plants may contain mercury in elemental, oxidized, and particulate forms. Elemental mercury in the exhaust gases does not stick to soot and other particles, but tends to remain in vapor form even after the exhaust gases are cooled to about 65° C. Therefore, elemental mercury in the exhaust gases is not recovered by conventional dust removal devices, such as, electrostatic precipitators, fabric filters, or conventional scrubbers, but is, instead, released into the atmosphere.
High mercury emissions in the exhaust gases from municipal solid waste incinerators are often regulated with powdered, activated carbon being injected into the exhaust gases upstream of the air pollution control devices. However, the level of mercury emissions per unit volume of flue gases from power plants is about one or two orders of magnitude lower than that emitted from waste incinerators. This makes it very difficult to capture such low mercury concentration levels from power plants by using the current activated carbon technology in a cost-effective manner.
Many fuels contain chlorine, which reacts with a portion of the mercury in the flue gases to form mercury chlorides. Gaseous mercury chlorides tend to condense on fly ash particles or on high surface area sorbents, which may effectively be removed from exhaust gases by conventional dust removal devices. Mercury chlorides are also highly soluble in water and, thus, they may be removed from the flue gas by absorption in the aqueous solutions of wet scrubbing units.
Early studies on trace elements released from coal combustion systems have shown that an increase in chlorine content in the furnace of the combustion systems leads to an increase in HgCl2 formation and that a spray dryer is effective in removing HgCl2 from the flue gas exiting the furnace. More recently, patents have disclosed mercury reduction methods to be used with specific flue gas cleaning equipment, which methods include increasing the Cl-content in the exhaust gas.
U.S. Pat. No. 5,435,980 discloses increasing the amount of chloride supplied to a spray dryer when cleaning flue gas that results from combusting coal having a low chloride content in order to convert elemental Hg to HgCl2. The chloride increase is performed by incorporating, e.g., an alkaline metal salt solution in the aqueous suspension of basic absorbent in the spray dryer, by supplying chlorine-containing material to the coal in the furnace or by injecting gaseous HCl into the flue gas downstream of the furnace. Alternatively, U.S. Pat. No. 5,900,042 suggests reacting a gas stream with, e.g., a chlorine solution or chloric acid (HClO3) to convert elemental mercury in the gas stream to soluble mercury compounds, and passing the gas stream through a wet scrubber.
European patent publication No. 0 860 197 suggests adding a mercury chlorinating agent, e.g., hydrogen chloride (HCl) or ammonium chloride (NH4Cl), to exhaust gas upstream of a catalytic NOx reduction unit to convert elemental mercury into mercury chloride (HgCl2) on the denitrating catalyst. In this method, the water-soluble HgCl2 is removed in a wet desulfurizing unit with an alkaline absorbing solution. This method is usable only in systems comprising a denitrating catalyst.
All the methods discussed in the patents referred to above, however, may suffer from poor mercury removal efficiency at low mercury levels and/or cause corrosion in the exhaust gas duct.